Amplifiers are used in numerous electronic circuits. One challenging environment is the amplification of the output of a photodiode or optical detector. The amplifier circuit is used to convert the current signal from the photodiode into a voltage signal. The current may have a wide dynamic range due to the variations in the input optical power. As a result, the amplifier circuit must accommodate a wide dynamic range. The prior art solutions generally use a trans-impedance amplifier and associated circuitry that changes the input impedance and the gain of the overall circuit. These solutions have the beneficial effect of being able to deal with the wide dynamic range. However, the variation in the input impedance result in different frequency responses for different gain settings. This results in inaccuracies in the amplified waveform.
Thus there exists a need for an amplifier circuit and method that has a variable gain and a constant input impedance.